Contralto is the deepest female classical singing voice, with the lowest tessitura falling between tenor and mezzo-soprano. It typically ranges between the F below middle C to the second G above middle C , although at the extremes some voices can reach the E below middle C or the second B above middle C. "Contralto" is meaningful only in reference to classical and operatic singing, as other genres lack a system of vocal categorization comparable to that generally accepted in the classical context. Even within current operatic practice, contraltos are often classed as mezzo-sopranos, because singers in each range can cover for those in the other. When appearing separately, the term "contralto" applies only to female singers; men whose voices fall in the same range or higher are known as "countertenors." The terms "contralto" and "alto" are not synonymous, the latter technically denoting a specific vocal range in choral singing without regard to factors like tessitura, vocal timbre, vocal facility, and vocal weight. Within the category of contraltos are three generally recognized subcategories - coloratura contralto, lyric contralto, and dramatic contralto - that usefully describe the voice type in general terms. Note, however, that they do not always apply with precision to individual singers; some exceptional dramatic contraltos, such as Ernestine Schumann-Heink and Sigrid Onégin, were technically equipped to perform not only heavy, dramatic music by the likes of Wagner but also florid compositions by Donizetti.